Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, the densest giant planet. Neptune is 17 times the mass of Earth more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU. It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier.
Its largest moon, was discovered shortly thereafter, though none of the planet's remaining known 13 moons were located telescopically until the 20th century. The planet's distance from Earth gives it a small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989; the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has allowed for additional detailed observations from afar. Like Jupiter and Saturn, Neptune's atmosphere is composed of hydrogen and helium, along with traces of hydrocarbons and nitrogen, though it contains a higher proportion of "ices" such as water and methane. However, similar to Uranus, its interior is composed of ices and rock. Traces of methane in the outermost regions in part account for the planet's blue appearance. In contrast to the hazy featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns.
For example, at the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h; because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the planet's centre are 5,400 K. Neptune has a faint and fragmented ring system, discovered in 1984 later confirmed by Voyager 2; some of the earliest recorded observations made through a telescope, Galileo's drawings on 28 December 1612 and 27 January 1613 contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky. At his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that day.
This apparent backward motion is created. Because Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope. In July 2009, University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was at least aware that the "star" he had observed had moved relative to the fixed stars. In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams began work on the orbit of Uranus using the data. Via Cambridge Observatory director James Challis, he requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams produced several different estimates of a new planet. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no enthusiasm in his compatriots.
In June 1846, upon seeing Le Verrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded Challis to search for the planet. Challis vainly scoured the sky throughout September. Meanwhile, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune within 1° of where Le Verrier had predicted it to be, about 12° from Adams' prediction. Challis realised that he had observed the planet twice, on 4 and 12 August, but did not recognise it as a planet because he lacked an up-to-date star map and was distracted by his concurrent work on comet observations.
In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who deserved credit for the discovery. An international consen
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus; the scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or 2.512. For example, a magnitude 1 star is 100 times brighter than a magnitude 6 star; the brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. Astronomers use two different definitions of magnitude: absolute magnitude; the apparent magnitude is the brightness of an object. Apparent magnitude depends on an object's intrinsic luminosity, its distance, the extinction reducing its brightness; the absolute magnitude describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars.
A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun. The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can be assigned to artificial objects in Earth orbit with the International Space Station sometimes reaching a magnitude of −6; the magnitude system dates back 2000 years to the Greek astronomer Hipparchus who classified stars by their apparent brightness, which they saw as size. To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a faint star such as Alcor. In 1736, the mathematician John Keill described the ancient naked-eye magnitude system in this way: The fixed Stars appear to be of different Bignesses, not because they are so, but because they are not all distant from us; those that are nearest will excel in Bigness.
Hence arise the Distribution of Stars, according to their Order and Dignity, into Classes. For all the other Stars, which are only seen by the Help of a Telescope, which are called Telescopical, are not reckoned among these six Orders. Altho' the Distinction of Stars into six Degrees of Magnitude is received by Astronomers, and among those Stars which are reckoned of the brightest Class, there appears a Variety of Magnitude. For Example: The little Dog was by Tycho placed among the Stars of the second Magnitude, which Ptolemy reckoned among the Stars of the first Class: And therefore it is not either of the first or second Order, but ought to be ranked in a Place between both. Note that the brighter the star, the smaller the magnitude: Bright "first magnitude" stars are "1st-class" stars, while stars visible to the naked eye are "sixth magnitude" or "6th-class"; the system was a simple delineation of stellar brightness into six distinct groups but made no allowance for the variations in brightness within a group.
Tycho Brahe attempted to directly measure the "bigness" of the stars in terms of angular size, which in theory meant that a star's magnitude could be determined by more than just the subjective judgment described in the above quote. He concluded that first magnitude stars measured 2 arc minutes in apparent diameter, with second through sixth magnitude stars measuring 1 1⁄2′, 1 1⁄12′, 3⁄4′, 1⁄2′, 1⁄3′, respectively; the development of the telescope showed that these large sizes were illusory—stars appeared much smaller through the telescope. However, early telescopes produced a spurious disk-like image of a star, larger for brighter stars and smaller for fainter ones. Astronomers from Galileo to Jaques Cassini mistook these spurious disks for the physical bodies of stars, thus into the eighteenth century continued to think of magnitude in terms of the physical size of a star. Johannes Hevelius produced a precise table of star sizes measured telescopically, but now the measured diameters ranged from just over six seconds of arc for first magnitude down to just under 2 seconds for sixth magnitude.
By the time of William Herschel astronomers recognized that the telescopic disks of stars were spurious and a function of the telescope as well as the brightness of the stars, but still spoke in terms of a star's size more than its brightness. Well into the nineteenth century the magnitude system
Penthesilea was an Amazonian queen in Greek mythology, the daughter of Ares and Otrera and the sister of Hippolyta and Melanippe. She assisted Troy in the Trojan War. In the five book epic Aethiopis, part of the Epic Cycle on the Trojan War, the coming to Troy of Penthesilea and Memnon was described in detail; the Aethiopis is attributed to Arctinus of Miletus. The main character of the epic is Achilles, who fights Penthesilea and Memnon before he is himself killed. Although Aethiopis has been lost, the Epic Cycle has been adapted and recycled in different periods of the classical age; the tradition of retelling the epic fall of Troy is indebted to Homer's Iliad and Odyssey, which were grounded in oral storytelling and were only written down when the Greek alphabet was adopted in ancient Greece. In the Aethiopis Penthesilea is a Thracian woman warrior, she was an daughter of Ares, who comes to help the Trojans. She arrived with 12 other Amazon warriors. After a day of distinguishing herself on the battlefield, Penthesilea confronts Achilles.
Achilles kills her. Thersites rebukes Achilles for having fallen in love. Thersites is killed by Achilles, who travels to the island of Lesbos to be purified before returning to Troy and fighting Memnon. According to Homer, the Trojan king Priam had fought the Amazons in his youth on the Sangarius River in Phrygia, some 350 miles east of Troy. Writers of the antiquity located Amazons geographically in Anatolia and started an epic tradition where Greek heroes, such as Heracles and Theseus, fought an Amazon warrior of distinction; the Aethiopis version of the Penthesilea legend has become known as the Homeric tradition. Different traditions of the Penthesilea legend appear to have existed in the time the Epic Cycle was published. In a lost poem of Stesichorus, believed to have been published in the 7th or 6th century, Penthesilea rather than Achilles had killed Hector. At the Temple of Apollo Epicurius, built in the mid- to late-5th century BC, scenes from the Trojan War are preserved in the Bassae Frieze, a high relief marble sculpture in 23 panels.
Here the Greek army is charged by the Amazons, who gain the upper hand, at the height of the battle Achilles slays Penthesilea on a slab known as BM 537. Achilles and Penthesilea are flanked by an Amazon. Penthesilea is identified as a queen by a crown. Penthesilea, shown on the ground just before being struck, Achilles are exchanging a gaze; the final slab of the series on the Amazons depicts a truce between the Greek army and the Amazons at the end of the battle. According to Pausanias, the throne of Zeus at Olympia bore a painting by Panaenus of the dying Penthesilea being supported by Achilles. Pausanias wrote "And, at the extremity of the painting, is Penthesilea breathing her last, Achilles supporting her"; the motive of Achilles supporting a dying or dead Penthesilea has been preserved at the Temple of Aphrodisias and was reinterpreted in sculptures and mosaics in ancient Rome. A black figure vase from about 510–500 BC shows Achilles carrying Penthesilea from the battlefield; the subject of Penthesilea was treated so by the so called Penthesilea Painter, active between 470 and 450 BC, that Adolf Furtwängler dubbed him "The Penthesilea Painter".
A considerable corpus for this innovative and prolific painter, whose work bridged the "Severe style" and Classicism and must have had a workshop of his own, was assembled in part by J. D. Beazley. In the Pseudo-Apollodorus Epitome of the Bibliotheke she is said to have been killed by Achilles, "who fell in love with the Amazon after her death and slew Thersites for jeering at him". In the 3rd century BC Lycophron went against the grain of the Homeric tradition; the poet had been born in Euboea, the site of a shrine to wounded Amazons who had fought in a mythic Battle for Athens. Lycophron tells the story of the young Amazon Clete, Penthesilea's attendant, left behind in Pontus. Clete sets out with a company of Amazons to search for Penthesilea when she does not return from the Trojan War; the ship with Amazons is swept of course and after a shipwreck on the toe of Italy in Bruttium, Clete becomes the queen of the Amazons that settle there. In Virgil's Aeneid, written between 29 and 19 BC, the Trojan army falls back.
Achilles drags the greatest Trojan warrior Hector around the city walls and sells his dead body to king Priam for gold. Penthesilea is cast as a tragic Amazon queen; when Aeneas sees the panel of Penthesilea in the Juno temple of Carthage, he knows that the defeat of Penthesilea and Memnon presage a chain of events that would culminate in the sacking of the city. Penthesilea's fate foreshadows that of Camilla, described in detail by Virgil in the epic. According to Virgil, Penthesilea is a bellatrix who dared to fight men. Virgil based his narrative in Homer's Iliad, while relying on the Epic Cycle for his portrayal of Penthesilea. Virgil reworked oral legends into an epic on the foundation of Rome. In Aeneid the Romans descended from the hero Aeneas and Trojan refugees who sailed to Italy after the Trojan War; this interweaving of the Penthesilea legend with the founding legend of Rome can be traced to Lycophron. In his universal history Bibliotheca historica Diodorus Siculus in the 1st century BC celebrated Penthesilea as the last Amazon to win renown for valour in war.
Diodorus wrote that after the Trojan War the Amazons diminished and tales of their former glor
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
The kilometre, or kilometer is a unit of length in the metric system, equal to one thousand metres. It is now the measurement unit used for expressing distances between geographical places on land in most of the world. K is used in some English-speaking countries as an alternative for the word kilometre in colloquial writing and speech. A slang term for the kilometre in the US and UK military is klick. There are two common pronunciations for the word; the former follows a pattern in English whereby metric units are pronounced with the stress on the first syllable and the pronunciation of the actual base unit does not change irrespective of the prefix. It is preferred by the British Broadcasting Corporation and the Australian Broadcasting Corporation. Many scientists and other users in countries where the metric system is not used, use the pronunciation with stress on the second syllable; the latter pronunciation follows the stress pattern used for the names of measuring instruments. The problem with this reasoning, however, is that the word meter in those usages refers to a measuring device, not a unit of length.
The contrast is more obvious in countries using the British rather than American spelling of the word metre. When Australia introduced the metric system in 1975, the first pronunciation was declared official by the government's Metric Conversion Board. However, the Australian prime minister at the time, Gough Whitlam, insisted that the second pronunciation was the correct one because of the Greek origins of the two parts of the word. By the 8 May 1790 decree, the Constituent assembly ordered the French Academy of Sciences to develop a new measurement system. In August 1793, the French National Convention decreed the metre as the sole length measurement system in the French Republic; the first name of the kilometre was "Millaire". Although the metre was formally defined in 1799, the myriametre was preferred to the "kilometre" for everyday use; the term "myriamètre" appeared a number of times in the text of Develey's book Physique d'Emile: ou, Principes de la science de la nature, while the term kilometre only appeared in an appendix.
French maps published in 1835 had scales showing myriametres and "lieues de Poste". The Dutch gave it the local name of the mijl, it was only in 1867 that the term "kilometer" became the only official unit of measure in the Netherlands to represent 1000 metres. Two German textbooks dated 1842 and 1848 give a snapshot of the use of the kilometre across Europe - the kilometre was in use in the Netherlands and in Italy and the myriametre was in use in France. In 1935, the International Committee for Weights and Measures abolished the prefix "myria-" and with it the "myriametre", leaving the kilometre as the recognised unit of length for measurements of that magnitude. In the United Kingdom, road signs show distances in miles and location marker posts that are used for reference purposes by road engineers and emergency services show distance references in unspecified units which are kilometre-based; the advent of the mobile phone has been instrumental in the British Department for Transport authorising the use of driver location signs to convey the distance reference information of location marker posts to road users should they need to contact the emergency services.
In the US, the National Highway System Designation Act of 1995 prohibits the use of federal-aid highway funds to convert existing signs or purchase new signs with metric units. The Executive Director of the US Federal Highway Administration, Jeffrey Paniati, wrote in a 2008 memo: "Section 205 of the National Highway System Designation Act of 1995 prohibited us from requiring any State DOT to use the metric system during project development activities. Although the State DOT's had the option of using metric measurements or dual units, all of them abandoned metric measurements and reverted to sole use of inch-pound values." The Manual on Uniform Traffic Control Devices since 2000 is published in both metric and American Customary Units. Some sporting disciplines feature 1000 m races in major events, but in other disciplines though world records are catalogued, the one kilometre event remains a minority event; the world records for various sporting disciplines are: Conversion of units, for comparison with other units of length Cubic metre Metric prefix Mileage Odometer Orders of magnitude Square kilometre Media related to Distance indicators at Wikimedia Commons
The ecliptic is the mean plane of the apparent path in the Earth's sky that the Sun follows over the course of one year. This plane of reference is coplanar with Earth's orbit around the Sun; the ecliptic is not noticeable from Earth's surface because the planet's rotation carries the observer through the daily cycles of sunrise and sunset, which obscure the Sun's apparent motion against the background of stars during the year. The motions as described above are simplifications. Due to the movement of Earth around the Earth–Moon center of mass, the apparent path of the Sun wobbles with a period of about one month. Due to further perturbations by the other planets of the Solar System, the Earth–Moon barycenter wobbles around a mean position in a complex fashion; the ecliptic is the apparent path of the Sun throughout the course of a year. Because Earth takes one year to orbit the Sun, the apparent position of the Sun takes one year to make a complete circuit of the ecliptic. With more than 365 days in one year, the Sun moves a little less than 1° eastward every day.
This small difference in the Sun's position against the stars causes any particular spot on Earth's surface to catch up with the Sun about four minutes each day than it would if Earth would not orbit. Again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun; the actual speed with which Earth orbits the Sun varies during the year, so the speed with which the Sun seems to move along the ecliptic varies. For example, the Sun is north of the celestial equator for about 185 days of each year, south of it for about 180 days; the variation of orbital speed accounts for part of the equation of time. Because Earth's rotational axis is not perpendicular to its orbital plane, Earth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about 23.4°, known as the obliquity of the ecliptic. If the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes.
The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. The crossing from south to north is known as the vernal equinox known as the first point of Aries and the ascending node of the ecliptic on the celestial equator; the crossing from north to south is descending node. The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26,000 years, a process known as lunisolar precession, as it is due to the gravitational effect of the Moon and Sun on Earth's equatorial bulge; the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, hence of the ecliptic, known as planetary precession; the combined action of these two motions is called general precession, changes the position of the equinoxes by about 50 arc seconds per year.
Once again, this is a simplification. Periodic motions of the Moon and apparent periodic motions of the Sun cause short-term small-amplitude periodic oscillations of Earth's axis, hence the celestial equator, known as nutation; this adds a periodic component to the position of the equinoxes. Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's equator with respect to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic, it is about 23.4° and is decreasing 0.013 degrees per hundred years due to planetary perturbations. The angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, from these ephemerides various astronomical values, including the obliquity, are derived; until 1983 the obliquity for any date was calculated from work of Newcomb, who analyzed positions of the planets until about 1895: ε = 23° 27′ 08″.26 − 46″.845 T − 0″.0059 T2 + 0″.00181 T3 where ε is the obliquity and T is tropical centuries from B1900.0 to the date in question.
From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated: ε = 23° 26′ 21″.45 − 46″.815 T − 0″.0006 T2 + 0″.00181 T3 where hereafter T is Julian centuries from J2000.0. JPL's fundamental ephemerides have been continually updated; the Astronomical Almanac for 2010 specifies:ε = 23° 26′ 21″.406 − 46″.836769 T − 0″.0001831 T2 + 0″.00200340 T3 − 0″.576×10−6 T4 − 4″.34×10−8 T5 These expressions for the obliquity are intended for high precision over a short time span ± several centuries. J. Laskar computed an expression to order T10 good to 0″.04/1000 years over 10,000 years. All of these expressions are for the mean obliquity, that is, without the nutation of the equator included; the true or instantaneous obliquity includes the nutation. Most of the major bodies of the Solar System o
In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans after Paris of Troy took Helen from her husband Menelaus, king of Sparta. The war is one of the most important events in Greek mythology and has been narrated through many works of Greek literature, most notably Homer's Iliad; the core of the Iliad describes a period of four days and two nights in the tenth year of the decade-long siege of Troy. Other parts of the war are described in a cycle of epic poems, which have survived through fragments. Episodes from the war provided material for Greek tragedy and other works of Greek literature, for Roman poets including Virgil and Ovid; the war originated from a quarrel between the goddesses Hera and Aphrodite, after Eris, the goddess of strife and discord, gave them a golden apple, sometimes known as the Apple of Discord, marked "for the fairest". Zeus sent the goddesses to Paris, who judged that Aphrodite, as the "fairest", should receive the apple. In exchange, Aphrodite made Helen, the most beautiful of all women and wife of Menelaus, fall in love with Paris, who took her to Troy.
Agamemnon, king of Mycenae and the brother of Helen's husband Menelaus, led an expedition of Achaean troops to Troy and besieged the city for ten years because of Paris' insult. After the deaths of many heroes, including the Achaeans Achilles and Ajax, the Trojans Hector and Paris, the city fell to the ruse of the Trojan Horse; the Achaeans desecrated the temples, thus earning the gods' wrath. Few of the Achaeans returned safely to their homes and many founded colonies in distant shores; the Romans traced their origin to Aeneas, Aphrodite's son and one of the Trojans, said to have led the surviving Trojans to modern-day Italy. The ancient Greeks believed that Troy was located near the Dardanelles and that the Trojan War was a historical event of the 13th or 12th century BC, but by the mid-19th century AD, both the war and the city were seen as non-historical. In 1868, the German archaeologist Heinrich Schliemann met Frank Calvert, who convinced Schliemann that Troy was a real city at what is now Hissarlik in Turkey.
On the basis of excavations conducted by Schliemann and others, this claim is now accepted by most scholars. Whether there is any historical reality behind the Trojan War remains an open question. Many scholars believe that there is a historical core to the tale, though this may mean that the Homeric stories are a fusion of various tales of sieges and expeditions by Mycenaean Greeks during the Bronze Age; those who believe that the stories of the Trojan War are derived from a specific historical conflict date it to the 12th or 11th century BC preferring the dates given by Eratosthenes, 1194–1184 BC, which corresponds with archaeological evidence of a catastrophic burning of Troy VII. The events of the Trojan War are found in many works of Greek literature and depicted in numerous works of Greek art. There is no authoritative text which tells the entire events of the war. Instead, the story is assembled from a variety of sources, some of which report contradictory versions of the events; the most important literary sources are the two epic poems traditionally credited to Homer, the Iliad and the Odyssey, composed sometime between the 9th and 6th centuries BC.
Each poem narrates only a part of the war. The Iliad covers a short period in the last year of the siege of Troy, while the Odyssey concerns Odysseus's return to his home island of Ithaca following the sack of Troy and contains several flashbacks to particular episodes in the war. Other parts of the Trojan War were told in the poems of the Epic Cycle known as the Cyclic Epics: the Cypria, Little Iliad, Iliou Persis and Telegony. Though these poems survive only in fragments, their content is known from a summary included in Proclus' Chrestomathy; the authorship of the Cyclic Epics is uncertain. It is thought that the poems were written down in the 7th and 6th century BC, after the composition of the Homeric poems, though it is believed that they were based on earlier traditions. Both the Homeric epics and the Epic Cycle take origin from oral tradition. After the composition of the Iliad and the Cyclic Epics, the myths of the Trojan War were passed on orally in many genres of poetry and through non-poetic storytelling.
Events and details of the story that are only found in authors may have been passed on through oral tradition and could be as old as the Homeric poems. Visual art, such as vase painting, was another medium. In ages playwrights and other intellectuals would create works inspired by the Trojan War; the three great tragedians of Athens—Aeschylus and Euripides—wrote a number of dramas that portray episodes from the Trojan War. Among Roman writers the most important is the 1st century BC poet Virgil. In Book 2 of the Aeneid, Aeneas narrates the sack of Troy; the following summary of the Trojan War follows the order of events as given in Proclus' summary, along with the Iliad and Aeneid, supplemented with details drawn from other authors. According to Greek mythology, Zeus had become king of the gods by overthrowing his father Cronus. Zeus was not faithful to his wife and sister Hera, had many relationships from which many children were born. Since Zeus believed that there were too many people populating the earth, he envisioned